PHOENIX ― After almost six hours of largely peaceful protests, police and demonstrators clashed outside the Phoenix Convention Center in Arizona following a campaign-style rally by President Donald Trump.

Tension between protesters and authorities intensified shortly after 9 p.m. local time, with police firing what appeared to be pepper spray projectiles, or pepper balls, in an effort to disperse the crowd. It was unclear what exactly had started the uproar.

Four people who were in the middle of the crowd said Trump supporters and opponents were yelling at one another as the crowd dispersed following the rally ― a rambling, 77-minute address.

“I was outside and nothing was happening when a cop just threw a smoke bomb and dispersed everybody,” Misha Smith, 42, said outside the convention center. “Then people started throwing water bottles.”

Not tear gas that police are firing to disburse Trump protests, it’s pepper balls. Here’s a shell that landed near me. pic.twitter.com/wJUzb5WI5J

After a tense standoff, police officers wearing gas masks stood in a line in front of the convention center on 2nd Street and ordered the crowd to leave before moving forward with riot shields. Loud bangs sounded in the sky.

“Everyone was just standing there,” said Jalyna Ramos, 23, a University of Arizona student. “We were literally just standing there, just hanging out. Literally out of nowhere people started running. It sounded like a really loud gunshot, and I saw a woman on the floor in tears.”

The confrontation dragged on for more than an hour as police pressed the remaining protesters a block away from the convention center, then lined up along the street. They repeatedly announced through a loudspeaker that anyone who remained in the area would be arrested.

Police then pushed into the remaining crowd, which had thinned to a few dozen people, and again launched projectiles. They pepper-sprayed a crowd that included a young girl.

Phoenix police said they did not carry out any arrests, and no one was reported injured.

But law enforcement officials said some protesters had thrown rocks and bottles at authorities, telling AZ Central in a statement that they had “dispersed some gas in the area.”

Thousands of people were expected to gather at the Phoenix venue that hosted Trump’s rally, with some of the president’s most ardent supporters lining up by Tuesday morning. At least 3,000 others said they planned to protest Trump and his anti-immigrant views outside the convention center in an effort to “send a very clear message that we will not pardon white supremacy.”